There are many applications in which surfaces of a hollow body are to be coated with material for various reasons. For example, the interior portions of light bulbs or similar lamps must often be coated with various substances to control light rays emitted from the bulb, and hollow body insulators are often coated on their interior surface with similar insulating material or the like. Similarly, various dishes, plates, pans and other items commonly used in the food preparation industry, domestic use, and the like, often require the application of specialized coatings or surface finishes such as non-stick material and the like to various parts of the products.
Methods previously employed to coat the inner surfaces of hollow bodies included dipping the product, or mechanical application of a coating material by a brush or similar applicator devices. Standard dipping procedures, however, inherently coated both the inner surface and the outer surface. Where coating of the outer surface was not desired, dipping procedures required tedious masking procedures or post-dipping removal of the unwanted coating from the outer surfaces.
Similarly, application of coatings to the inner surfaces of hollow bodies by means of brushes or similar mechanical applicator devices required that the interior area of the hollow body be of sufficient size and shape to admit the applicating device and to enable its manipulation therewithin. Such application was inefficient and, in some instances, entirely unworkable due to the size and/or shape of the hollow body to be coated.
Likewise, spraying techniques were insufficient to overcome many of the deficiencies of these previous coating systems, as selective application of the coating was difficult without again involving tedious masking operations or subsequent overspray removal procedures.
One attempt to address the inadequacies of these previous coating methods and apparatuses is set forth in U.S. Pat. No. 1,621,016, which issued Mar. 15, 1927 to R. P. Jackson. The Jackson patent discloses the use of a tank or receptacle filled with liquid to be coated onto the inner surface of a hollow article. The tank has a lid member which is specifically designed to support the hollow body to be coated in an inverted position. A tubular member mounted within the tank has an open upper end located within the interior portions of the inverted hollow body and communicates with a vacuum pump at its distal end. The lid member supports the hollow body such that the surface of the liquid creates a seal around the mouth of the hollow body whereby a vacuum can be created within the hollow body to cause the liquid level therewithin to rise to a predetermined level. Thereafter, the vacuum is removed and the liquid level recedes to its initial position, leaving a coating on the inside surface of the hollow body. The Jackson apparatus and process, however, requires a lid member specifically designed to appropriately support the hollow body to be coated, and a separate lid structure must be provided for each particular product to be coated. Moreover, the rising level of the coating liquid within an inverted hollow body is controlled only by the pressure differential induced by the vacuum tube, making control of the coating process difficult, relatively unreliable, and totally dependent upon accurate control of the vacuum pressure and timing.
A method for coating the interior surface of a can structure is shown in U.S. Pat. No. 2,067,922 which issued Aug. 2, 1934 to J.M. Hothersall. The Hothersall process concerns the coating of can structures which are open at either end, and which can be momentarily sealed within the apparatus to form part of a coating chamber therewithin. A vacuum draws liquid upwardly within the coating chamber and the can, completely filling the can and thereby coating its inner surfaces. Rising fluid within the coating chamber eventually closes off the vacuum source by pushing a flow valve upwardly. The Hothersall process does not provide a viable method for selectively coating the interior surface of an inverted structure such as a pan or cooking utensil, as it relies upon the open-ended nature of the structure to be coated to form part of its coating chamber. Moreover, the Hothersall process is inefficient in that it requires complete filling of its coating chamber, and lacks precise control of what portions of the inner surfaces of a product are to be coated.
U.S. Pat. No. 2,692,209 which issued on Apr. 29, 1953 to A. Binder, Jr., describes a coating process for light bulbs wherein liquid is pulled into the bulb by a vacuum, and the vacuum is shut off when the liquid surface level reaches the desired uppermost height therewithin. The Binder reference also contemplates a need for removing the coating on the lower portions of the bulb where coating material is not desired. Again, the Binder apparatus relies upon precise control of the vacuum or pressure within the item to be coated to limit the surfaces being coated by the rising liquid. This requirement for precise pressure control is unreliable and inefficient, as differences in viscosity of the coating fluid, size and volume of the product being coated, temperature, and similar variables can cause unpredictable variations in the coating procedures. As a result, this process cannot adequately provide positive control of the coating process and precisely uniform results, as is required in many modern-day applications.
Consequently, while coating processes and apparatuses have been widely used for various products through the years, heretofore there has not been available a coating process which could simply, accurately and reliably, automatically and selectively coat a predetermined area of the inner surface of a hollow body. Moreover, there has not been available an apparatus or process which can be easily adapted to a wide variety of shapes and sizes of hollow bodies, and which can selectively coat predetermined areas of the inner surfaces of those hollow bodies with uniform consistency and reliability.